1. Field of the Invention
This invention relates to a head tracker system and more especially, although not exclusively, to a head tracker system for use in an aircraft in which the pilot is provided with a helmet-mounted display.
2. Discussion of Prior Art
Head tracker systems are well known and operate to determine a user""s head orientation and position relative to a fixed datum. Originally these systems were developed for use in military aviation but have recently found applications in virtual reality systems.
Since the earliest days of military aviation, pilots have not surprisingly preferred to navigate and aim weapons whilst looking up and out of the cockpit. This led to the evolution of the head-up display (HUD) which displays useful symbology which is appropriately referenced to the outside world (often termed xe2x80x9cspace stabilisedxe2x80x9d). HUDs typically have a viewing angle of thirty degrees or less and can consequently only be viewed when the pilot is looking in a generally forward direction. To increase the field of regard (that is the total volume of space over which the pilot can view the symbology and includes the pilot moving his head), helmet-mounted displays (HMDs) have evolved which essentially comprise a HUD which is mounted on the pilot""s helmet within his field of view. In order that ground stabilized symbols or imagery are presented to the pilot in the correct orientation with respect to the outside world, the symbol generator of the HUD must know in all three axes, that is elevation, azimuth and roll, where the pilot""s head is directed. This is achieved by determining (a) the angular orientation of the pilot""s head with respect to the aircraft axes and (b) the orientation (attitude) of the aircraft with the outside world. The former requirement has led to the need for a head tracker system. HMDs operate in conjunction with the head tracker system which determines the angular orientation of the user""s head with respect to the aircraft axes to ensure that the displayed information is correctly aligned in space or is accurately superimposed against objects in the outside world. For example, in the case of military aviation it is essential that a weapon aiming marker is accurately aligned over the target.
Typically, a head tracker system comprises a head mounting, most commonly a helmet, which is attached to the user""s head and a sensor system for determining the angular orientation of the helmet relative to a fixed reference datum. Although strictly speaking a head tracker system actually tracks the orientation of the head mounting and not the user""s head, it provides an accurate measure of the user""s head orientation provided the head mounting remains in a fixed orientation relative to the user""s head.
The reference datum is typically three axes which pass through a known point on the vehicle or airframe. A number of sensor systems have been proposed. For example, early tracking systems used mechanical linkages between the helmet and the vehicle and, whilst such systems were relatively accurate, they were cumbersome, restricted the user""s movement and posed particular problems during ejection from an aircraft. Further systems include optical sensor systems in which the helmet carries a number of light emitting diodes operating in the infra-red which are detected by position sensitive detectors which are mounted to the vehicle cockpit. Other known optical systems use distinctive physical markings which are detected by a camera. Magnetic sensor systems are also known in which an alternating or pulsed magnetic field is detected by sensors on the helmet. Inertial systems have been proposed which use gyroscopes and accelerometers; and hybrid systems which involve a combination of two or more of the above systems.
A particular requirement of a head tracker system for use in military aviation is high accuracy coupled with a fast dynamic response as the pilot""s head movements are often extremely rapid; typically these can be greater than two hundred degrees per second. These two qualities are often mutually exclusive and although advances in sensor technology, for example solid state gyroscopes, have increased the dynamic response of the tracker system, the accuracy of some of these systems is limited by drift over extended periods of operation.
It is known to initially set or calibrate, often termed boresighting, the head tracker system before the commencement of operation. For example in the case of military helicopters it is known to provide one, or sometimes two, dedicated boresighted reticle units (BRU) at known lines of sight. The BRU displays a reticle which is presented as a collimated image, often in the form of a circle and cross and which has a narrow viewing angle typically of the order of two degrees. Since the BRU produces a substantially collimated image, the image is essentially presented at infinity and is consequently only visible when the user""s head orientation (more particularly that of the helmet) is in a given alignment to the BRU axis. When the pilot boards the helicopter and dons the helmet and HMD he will depress a button to activate a boresight mode of operation. In the boresight mode of operation, the BRU is activated and the suitable boresighting symbology is displayed on the HMD as a space stabilised image. The pilot then uses a hand control to adjust the HMD symbology until it overlaps the BRU. At this point the pilot""s eye, HMD symbology and BRU marker are coincident and the system calculates an appropriate offset which is used in subsequent operations in conjunction with the head tracker output to position the HMD symbology. When a second BRU is provided the pilot repeats the procedure, thus providing the tracker system with two known starting conditions. The head tracker system and HMD are only boresighted at the commencement of operation and the BRU is thus of no further use and is consequently often de-activated.
In the case of a fixed wing aircraft which is provided with a HUD it is known to use the HUD to generate the boresight reticle in place of the BRU. Although a HUD will have a larger viewing angle, typically thirty degrees, it still produces a highly collimated image and can consequently be used to accurately orientate the head mounting.
Whilst initial boresighting ensures that the tracker system and HMD operate accurately at the commencement of operation, it cannot compensate for any mechanical misalignment of the HMD with respect to the helmet, nor for any drift in the tracker system. Furthermore whilst the boresighting process may not be lengthy this can still be regarded as an unacceptable delay before the vehicle is operational.
At present HMDs do not achieve the accuracy of HUDs even when operating on the boresight axis due to the limited accuracy of the head tracker which is due in part to drift. Moreover the distributed architecture of HMDs does not easily provide a high level of integrity, that is the assurance that it is providing accurate information. In the case of HMDs which use a binocular display, cross monitoring between the respective channels provides a degree of integrity. However in the case of the head tracker there is no independent parallel path by which the accuracy of the tracker can be verified. The present invention has arisen in an endeavour to provide a head tracker system which, at least in part, overcomes the problems of the known head tracker systems.
According to the present invention a head tracker system for determining a user""s head orientation relative to a datum comprises: a head mounting for attachment to the user""s head and a sensor system for sensing the orientation of the head mounting relative to the datum; characterised by a distinguishable marking and an optical sensor designed in use to be fixed respectively relative to a first known point fixed relative to the head mounting and a second known point fixed relative to the datum or vice versa and processing means for determining when the marking is within the field of view of the optical sensor and wherein the output of the processing means is used to correct for drift in the sensor system or to provide an independent verification that the system is functioning within preselected tolerances.
A particular advantage of a head tracker in accordance with the invention is that any drift in the system can be automatically compensated for each time a marking is detected within the field of view of the optical sensor. In addition to automatically compensating for drift the invention can further eliminate the need for an initial boresight alignment and therefore substantially reduce any delay before the system is initially fully operational. Alternatively or in addition to correcting for drift the present invention provides an independent assurance that the head tracker is functioning within prescribed tolerances. This is particularly important in applications where a further system such as a weapon aiming or infra red night vision system is reliant upon accurately knowing the user""s head orientation.
The distinguishable marking can comprise a spatial pattern and/or can be defined in part at least by its colour. The only requirement of the marking is that it can be unambiguously recognised and is indicative of a known position and orientation of the head mounting relative to the fixed datum. In one embodiment it is envisaged that the marking comprises features of the environment around the user such as, in an aircraft, part of the cockpit structure or instrumentation. To increase the visibility of the marking it is preferred for it to be defined using retro-reflective material.
Alternatively the head tracker further comprises a marking generator for generating the distinguishable marking. With such a generator the colour of the marking is conveniently defined by the wavelength of the light produced by the generator. Furthermore the marking can be defined as a temporal variation by modulating the light produced by the marking generator. It could also be defined by a combination of two or more of the above three techniques for example the marking could comprise a particular shape, having a particular wavelength which is modulated on and off at a selected frequency.
In a particularly preferred implementation the visual marking is a collimated image having an axis which is predetermined and which passes through said associated known point. With such a marking the optical sensor, which preferably comprises a video camera, is focussed at infinity. A particular advantage of such an arrangement is that, since the marking is only visible when the axis of the field of view of the optical sensor and the marking are substantially coincident, the processing means knows the orientation of the head mounting relative to the known datum whenever it detects the presence of the marking.
Advantageously the first and second known fixed points are selected such that the marking is within the field of view of the sensor whenever the user""s head is oriented in a direction in which it is most commonly directed during operation, typically this direction will be a substantially forward facing direction. A particular advantage of this arrangement is that it maximises the number of times the marking will be within the field of view of the optical sensor and hence maximises the frequency at which drift can be compensated for and/or verification of the system can be performed. Preferably when the optical sensor is associated with the first fixed point, that is the head mounting which conveniently comprises a helmet, the axis of the field of view of the sensor is substantially aligned with the forward looking direction of the user such that the output from the optical sensor is representative of the user""s view. Such information can for example in the case of an aircraft be recorded and subsequently used to evaluate the pilot""s performance. In a particularly preferred embodiment the marking generator comprises a boresight reticle unit. Advantageously when the head tracker system is for use in an aircraft or other vehicle which includes a head-up display the distinguishable marking is generated using the head-up display.
Alternatively when the marking is not a collimated image, such as for example a physical marking, an uncollimated light source or array of such sources, the optical sensor is focussed at a distance which corresponds with the expected distance between the sensor and the marking. Since the marking will be visible over a larger part of the field of view of the sensor the processing means is operable to detect the position of the marking within the field of view of the sensor in order to determine the orientation of the head mounting. To reduce the amount of processing the processing means is preferably operable to detect whenever the marking is within a selected portion of in the field of view of the sensor, such as for example within the centre position.
Advantageously the optical sensor comprises a video camera, such as a charge coupled device. Preferably the head tracker system further comprises at least two known fixed points having a respective distinguishable marking associated with it.
In a preferred embodiment the processing means comprises a correlator operable to correlate scene data captured by the optical sensor with data which is representative of the or each marking to determine if the or each marking is within, and/or where it is within, the field of view of the optical sensor.